One type of rotor aircraft has wings, a rotor and a means of forward thrust other than the rotor. The rotor provides most of the lift during short or vertical takeoffs, slow speed flight, and landings, and the wings provide most of the lift during cruise conditions. For a short or zero roll takeoff, the pilot pre-rotates the rotor to a selected speed. While pre-rotating, the pilot maintains the collective pitch near zero. The collective pitch refers to the angle of attack of the blades relative to the plane of rotation. At zero collective pitch, the leading and trailing edges of the blades are in a common plane with the plane of rotation.
When ready for liftoff, the operator releases the clutch driving the rotor so it freewheels and increases the collective pitch, which tilts the leading edge of each blade upward relative to the trailing edge. The rotor has tip weights to provide high inertia, and the inertia drives the rotor, which causes the aircraft to lift while a thrust means propels the aircraft forward. The operator gradually reduces the collective pitch as the aircraft picks up speed. The operator also decreases the aft tilt of the rotor, which reduces the air stream flowing through the rotor, thus decreasing the rotational speed. At cruising speeds and at an advance ratio greater than about 0.7, the collective pitch is generally between 1.5 and minus 0.5 degrees, and the rotor provides very little of the lift. The air stream flowing through the rotor causes the rotor to auto-rotate at a selected slow rotational speed. When landing, the operator tilts the rotor aft, which causes the rotor to speed up. The operator also increases the collective pitch when landing, causing the rotor to assume more of the lift required for the aircraft.
At high aircraft velocities, the rotor speed must be limited so that the tip velocity of the advancing rotor does not exceed the speed of sound. Because of this problem, the ratio of aircraft forward speed to rotor tip speed relative to the aircraft, known as the “advance ratio” or Mu, is limited to about 0.5 in helicopters and autogyros. A gyroplane as described above is able to achieve higher aircraft speeds by unloading the rotor and auto-rotating the rotor at a slow rotational speed, thus allowing the Mu to increase beyond that of conventional helicopters and autogryos.
Even though the rotor provides very little lift at cruising speeds, the advancing and retreating blades must provide equal lift moments about the rotor head. The advancing blade can only provide as much lift moment as the retreating blade. Once the rotor has been sufficiently unloaded by providing lift with the wings and propulsion by a source such as a propeller, the rotor blades continue to maintain lift moment equilibrium about the hub with rotor flapping. Rotor flapping is a mechanism by which the advancing and retreating blades can produce the same lift moments. In order to work, the blades must be free to pivot up and down relative to the hub. This free flapping allows the advancing blade, which if it has more lift due to a higher velocity across it than the retreating blade, to rise or flap up. As the advancing blade rises, the resultant flow angle across the blade drops and reduces its lift. The opposite occurs on the retreating blade. As the advancing blade goes up, the retreating blade drops since the blades are tied together and because the retreating blade is not producing as much lift as the advancing blade. As the retreating blade drops, the resulting angle of airflow across the blade goes up and increases its lift. The rotor will automatically increase flapping until the lift moments on the advancing and retreating blades are the same. This characteristic whereby the lift on the retreating blade increases as the blade drops works whether the air flows from the leading edge to the trailing edge or from the trailing edge to the leading edge And what allows the rotor to operate at advance ratios greater than 1.
Nevertheless, for stability, the rotor flapping must be kept within a selected range, such as about 1 to 4 degrees. U.S. Pat. No. 6,435,453 discloses that varying the collective pitch can control flapping. Decreasing the collective pitch decreases flapping. However, measuring flapping during flight is difficult because the rotor plane of rotation changes in a banked turn and because of rapid changes in air speed or gusts.
As the rotor RPM slows, the centrifugal force decreases until at some point there would not be enough centrifugal force to keep the relatively flexible rotor blades stable. Weight is added to the blade tips to allow the rotor to be slowed down as much as practical. U.S. Pat. No. 5,627,754 discloses that rotor RPM can be varied by tilting the rotor plane of rotation relative to the air stream and used a cyclic mixing arrangement whereby the length of the links controlling the rotor fore and aft movement could be varied. Instead of a cyclic mixing arrangement used in the previous patent to tilt the rotor, a tilting mast can be used once the rotor has been unloaded for high Mu flight.
Flying a gyroplane as described requires the operator to adjust the collective pitch from near zero, during pre-rotation, to a high level for takeoff and landing, and again to between 1.5 and minus 0.5 degrees at cruising speeds. Some skill must be acquired in order to properly adjust the collective pitch, and the operator must remain aware of the necessity of making the pitch changes. Also the operator must keep the rotor RPM from going too low whereby there would not be enough centrifugal force to the keep the rotor stable. In order to reduce the pilot workload and make the aircraft easier to fly, both the rotor flapping and RPM can and should be controlled automatically.